The advent of composite pipes, exhibiting the capability to be mechanically joined expeditiously in sections to provide permanently sealed connections, has supplanted welded steel pipe for many fluid transport applications. In particular, it has been proven that pipe engineers no longer need to depend exclusively upon welded steel pipe as the most reliable and economical system for transporting various fluids, such as water, oil, gas and slurry products. In addition to features, such as high strength-to-weight ratio and long term resistance to cyclic fatigue and corrosion, composite pipes exhibit an extremely smooth inner surface that functions to reduce fluid flow friction to thus lower pumping costs.
The most important aspect for drawing economic comparisons between steel and composite pipes of equivalent linear footage, constitutes the method and resulting labor costs required to join and seal pairs of longitudinally connected pipe sections together. Steel pipe sections are most economically joined and sealed by welding, rather than by the use of sealed bolted flanges or threaded couplings. Conversely, composite pipe sections are most economically joined and sealed together by the use of mechanical couplings, rather than by the use of in-the-field bonded connections. The speed and ease by which composite pipe sections can be coupled together and sealed, as well as uncoupled for repair or replacement purposes, greatly enhances the economic worthwhile of composite pipes for many fluid transport systems.
Various federal, state and local governmental agencies, such as the United States Environmental Protection Agency (EPA), now require that pipelines conveying hazardous liquids or gases provide secondary containment capabilities in the event of leakage. One conventional approach to solving the secondary containment problem is to install pipelines in a trench, having an impermeable liner. Leak-proof trenches of this type are expensive to install, difficult to maintain and cannot be used when a pipeline traverses rivers or lakes or are installed along the floor of an ocean.
Another conventional method for solving the secondary containment problem is to utilize a double-wall pipe, comprising impermeable inner and outer walls separated radially by an annular void or permeable annulus structure. Leakage is continuously monitored by placing leak detecting sensors at strategic locations within selected pip sections. Double-wall pipes of this type are designed to resist normal longitudinal and circumferential stresses imposed on the pipe by fluctuating fluid pressures and flow velocities However, such pipes are incapable of efficiently resisting other types of extreme pressures and bending and compression loads imposed on the pipes when they are put into commercial use.
Since the outer wall of a standard double-wall pipe has a diameter larger than that of the inner wall, the outer wall will experience hoop stresses of higher magnitude than the inner pipe for a given working pressure. Therefore, it is common in the industry to construct the outer wall to be at least as thick as the inner wall, which serves as the primary fluid transport container. In some pipe applications, the inner and outer walls are separated by structural members, such as corrugated sheets, longitudinal or circumferential ribs or spokes, clips or permeable rigid foam materials, in an attempt to increase the overall structural integrity of the pipe.
For example, conventional double-wall composite pipes of this type are disclosed in U.S. Pat. Nos. 3,784,441 and 4,758,024. In particular, the double-wall composite pipes disclosed in these patents comprise impermeable inner and outer walls separated by ribs. The composite load-resisting material comprising the inner and outer walls of the pipes usually comprises an impermeable fiber-reinforced thermosetting resin.
The annulus region of a conventional double-wall composite pipe, disposed radially between the inner and outer walls of the pipe, is primarily designed to provide for the secondary containment of fluids This region, although housing structural members of the type described above for certain commercial pipe applications, is generally non-structural in physical make-up. In particular, the region between the pipe's walls is normally sealed and evacuated, filled with a liquid to disclose a leak in either pipe wall or remains air-filled and houses leak-detecting sensors or probes. Such an annulus region, fabricated separately apart from the fabrication of the inner and outer walls of the pipe, is usually unduly complicated in physical make-up and expensive to fabricate, install and service.
The joined sections of conventional double-wall composite pipes are normally secured together at joint connections by an adhesive or by bolted flange connections. Fabrication of these types of joint connections is oftentimes found to be unduly labor intensive, difficult to achieve expeditiously and efficiently, and cost prohibitive. The cost factor is compounded due to the complex and expensive production equipment required to individually fabricate components of such conventional joint connections.
Further, standard double-wall composite pipes do not possess hydrostatic design basis strengths greater than 12,000 psi, as dictated by ASTM D2992, primarily since the strength at their joint connections does not exceed the interlaminar shear or tensile strength of the composite matrix material composing the pipes Conventional pipes of this type also possess relatively high longitudinal strain values and, consequently, will tend to elongate excessively, when placed into use. Elongation of the pipes will produce buckling stresses that must be resisted by either burying the pipes underground or by utilizing specially designed pipe anchoring devices. Expansion loops or special compensating devices are also used to compensate for pipe expansion due to changes in pipe material temperature and/or longitudinal stress.
Coupling structures, used at the joint connections for the pipes to connect and seal the inner walls together, do not also connect and seal the outer walls of adjacent pipe sections together. Thus, the structural integrity of the integrated pipes is less than desired. Further, no permeable stress-resisting structure is provided between the impermeable inner and outer walls. Also, the pressure and flow rate of fluid leaking from a fractured inner wall is generally not inhibited within the pipe.